multiple file package changes in blender 2.7?
I made the example of wiki blender with a little change. It seems to work well.
Version 2.79

# To support reload properly, try to access a package var, 
# if it's there, reload everything
if "bpy" in locals():
    import importlib
    if 'simple_obj_import' in locals():
        importlib.reload(simple_obj_import)
    if 'simple_obj_export' in locals():
        importlib.reload(simple_obj_export)

import bpy
from bpy.props import *
#from io_utils import ExportHelper, ImportHelper
from bpy_extras.io_utils import ExportHelper, ImportHelper